Conventionally, a sheet metal-made back face pulley which causes a cylindrical portion to be in contact with the back face of a belt to function as, for example, a tensioner is known. The applicant of the present application has disclosed a production method in which the length of the cylindrical portion is increased (elongated) (Patent Literature 1).
In the production method in which the length of the cylindrical portion is increased (elongated), as shown in the left half of FIG. 8, a sheet metal-made cup-shaped material 4 comprising: a circular base plate portion 2 from which a boss portion 1 is projected; and a cylindrical portion 3 which extends from an outer peripheral portion of the base plate portion 2 in one direction along the axis C1 of the base plate portion 2 is prepared. A rotary lower mold 5 is fitted from a lower open-end portion of the cup-shaped material 4, and a shaft portion 51 of the mold is fitted into the boss portion 1. A downward expansion 21 which is formed in the base plate portion 2 is fitted into a recess 52 of the rotary lower mold 5, so that the cup-shaped material 4 is set to the rotary lower mold 5 so as to be radially immovable, and a rotary upper mold 6 is opposed to the upper side of the rotary lower mold 5 via the base plate portion 2.
Then, the rotary upper mold 6 is lowered as indicated by the arrow e in the right half of FIG. 8, and a downward expansion 61 of the mold is fitted to the upper face of the downward expansion 21 of the base plate portion 2, so that the base plate portion 2 of the cup-shaped material 4 is clampingly held by the upper end face of the rotary lower mold 5 and the lower end face of the rotary upper mold 6. Thereafter, at least one of the rotary lower mold 5 and the rotary upper mold 6 is rotated about the axis C1 to rotate the rotary lower mold 5, the rotary upper mold 6, and the cup-shaped material 4 about the axis C1. In this way, while rotating the cup-shaped material 4, a first roll-forming roller 7 is moved in the direction of the arrow f, and a portion corresponding to an approximately lower half of the cylindrical portion 3 is clampingly pressed by a pressing face 71 which protrudes radially outward, and the outer circumferential face 53 of the rotary lower mold 5. This causes the portion corresponding to the approximately lower half of the cylindrical portion 3, to be downward extended as indicated by the reference numeral 8 while the approximately lower half of the cylindrical portion 3 is thinned.
As shown in the left half of FIG. 9, next, the cup-shaped material 4 in which the portion 8 corresponding to the approximately lower half of the cylindrical portion 3 is downward extended while being thinned is set to a rotary lower mold 5a so as to be radially immovable, and a rotary upper mold 6a is opposed to the upper side of the rotary lower mold 5a via the base plate portion 2. In this case, the rotary lower mold 5a which is slightly smaller in diameter than the rotary lower mold 5 used in FIG. 8 is used. Therefore, a small gap s is formed between the outer circumferential face 53a of the rotary lower mold 5a and the cylindrical portion 3. A cutaway 62 which allows ingress of an outer peripheral edge of an upper end portion of a second roll-forming roller that will be described later is formed in an outer peripheral edge portion of the rotary upper mold 6a. In the rotary lower mold 5a and rotary upper mold 6a which are shown in FIG. 9, portions corresponding to those of the rotary lower mold 5 and rotary upper mold 6 which are shown in FIG. 8 are denoted by the same reference numerals.
Then, the rotary upper mold 6a is lowered as indicated by the arrow g in the right half of FIG. 9, and a downward expansion 61 of the mold is fitted to the upper face of the downward expansion 21 of the base plate portion 2, so that the base plate portion 2 of the cup-shaped material 4 is clampingly held by the upper end face of the rotary lower mold 5a and the lower end face of the rotary upper mold 6a. Thereafter, at least one of the rotary lower mold 5a and the rotary upper mold 6a is rotated about the axis C1 to rotate the rotary lower mold 5a, the rotary upper mold 6a, and the cup-shaped material 4 about the axis C1. In this way, while rotating the cup-shaped material 4, a second roll-forming roller 9 is moved in the direction of the arrow h, and a thick portion 3a (see the left half of FIG. 9) corresponding to an approximately upper half of the cylindrical portion 3 is clampingly pressed by a pressing face 91 which is recessed radially inward, and the outer circumferential face 53a of the rotary lower mold 5a. This causes the thick portion 3a to be downward extended as indicated by the reference numeral 10 in the right half of FIG. 9 while being thinned, with the result that the cylindrical portion 3 in which the outer circumferential face is flat can be produced.
Namely, by the cup-shaped material 4 in which the thickness t1 of the cylindrical portion 3 shown in FIG. 10 is large and the axial length h1 of the cylindrical portion 3 is small, the cylindrical portion 3 in which, as shown in FIG. 11, the thickness t2 is small and the axial length h2 is larger than the axial length h1 in FIG. 9 can be produced.    Patent Literature 1: WO98/05447